Method for forming a joint using a self-piercing rivet

ABSTRACT

A joint is formed in a stack of at least two sheets of light metal alloy, using a self-piercing rivet that is fully hollow. The rivet is coated at least along a portion of its bore by a lubricant and pierces the upper surface thereof and such that the shank deforms outwardly to interlock with the materials but without penetration to the die side of the material. The outside diameter of the shank of the rivet is 5.4 mm or less. The die has a volume that is less than 60% or 70% of the effective solid volume of the rivet.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/414,970, filed on Jan. 15, 2015, currently pending, which is a U.S.National Stage Application of International Application No.PCT/GB2013/051879, filed Jul. 15, 2013, which claims priority fromUnited Kingdom Patent Application No. 1212631.4, filed Jul. 16, 2012,and United Kingdom Patent Application No. 1212844.3, filed Jul. 19,2012. The entirety of all the above-listed applications are incorporatedherein by reference.

The present invention relates to a method for forming a joint with aself-piercing rivet whereby the rivet is inserted into sheet materialwithout full penetration such that a deformed end of the rivet remainsencapsulated by an upset annulus of the sheet material.

A self-piercing rivet generally has a head and a partially hollow shank.It is driven by a punch into the sheet material such that it pierces thetop sheet or sheets and forms a mechanical interlock with the bottomsheet with the head often (but not always) flush with the upper surfaceof the top sheet. Since the bottom sheet is not pierced, through beingformed into a die, there is a reduced risk of corrosion occurring in thecompleted joint. Using self-piercing rivets in a joining process reducesthe number of production steps as compared to conventional riveting inwhich a hole first has to be drilled into the sheet material before therivet is inserted and then its projecting ends upset.

Self-piercing riveting, often in combination with adhesive, has beenused to great commercial success in the automobile industry wherelight-weight materials, such as aluminium, have been adopted for vehiclebody panels and other components in the interests of weight reductionand therefore reduced energy consumption. It is difficult, or notfeasible, to spot weld aluminium, particularly to steel, owing to itshigh thermal conductivity, low melting range and propensity to formoxide surface film.

More recently in the automotive industry there has been a move towardsusing high strength sheet metals. Our European patent no. 2024651describes a self-piercing rivet particularly suitable for joining highstrength, thick stack steels. Since such steels have a high ultimatetensile strength the insertion forces applied to the rivet arenecessarily high there is thus a significant risk of rivet collapse. Therivets must be heat treated to give them a medium/high hardness value(e.g. 450-580 Hv) so that they have sufficient strength. It has beenestablished that such a rivet is not always suitable for use with thickstack, high strength light metal alloys such as magnesium and aluminiumalloys especially where the material combination may have three or morelayers. Moreover, other conventional rivets are not generally suitablefor joining such materials.

Aluminium alloy sheet material generally exhibits superior ductility andso dies with relatively deep cavities tend to be used. However when suchdies are used with stronger materials such as high strength aluminium ormagnesium alloys the joints suffer from the tendency for the middlesheets to push through the lowermost sheet in the insertion process.This leaves a weakened joint which is often prone to corrosion and itmay not be possible to produce a satisfactory joint repeatedly in a massproduction environment.

Thick stack, light metal alloys such as magnesium or higher strengthaluminium exhibit relatively low ductility and thus have a tendency tofracture when riveted using self-piercing rivets and conventional dies.Components comprising thick stack joints, e.g. automobile shock towersor crash members, are sometimes produced by casting or extrusion, thisbeing a more economical and/or efficient in terms of design and/ormanufacture. The “button” of material that is deformed into the dietears or cracks during the rivet insertion process. This is undesirableas the finished joint is weakened and prone to corrosion. Moreover, ithas been found that there is a tendency for the end of the rivet shankto drag sheet material down through the joint to such an extent that itis pushed through and out of the lowermost sheet, resulting in a jointthat is prone to corrosion and may also be considered unacceptable inaesthetic appearance. In the process of being dragged down the sheetmaterial thins excessively and is prone to tearing or cracking. Thisresults in a loss of interlock and reduces the strength of the joint.Furthermore, the sheet material that has been dragged down may wraparound the piercing end of the rivet shank rather than being penetrated.This results in reduced peel strength. A die with a relatively smallvolume die cavity (i.e. a shallow die) may be used in order to avoidtearing or cracking. However, a reduction in the die depth serves toincrease the force experienced by the rivet during insertion, therebysubjecting the joint, die and riveting tool to greater stress. This isundesirable for the rivet in that it may cause buckling or fracturegiving the finished joint an inconsistent or unacceptable form qualityor strength. Reduction in die depth or other alterations to the dievolume or form, especially with the corresponding increased force, mayalso result in greater tendency for material from the lower stackcomponent to become detached and remain in the die as the joinedcomponent is removed from the die as there is a tendency for the die togrip the component as a result of the compacting of the material in thedie by the riveting process. This tendency may be exacerbated if tearingor cracking has been initiated during the rivet insertion process but itmay occur even without the initiation of such tearing or cracking.Whilst countermeasures such as polishing or die surface treatments arepossible to address the problem of retained material, the increasedstress imparted by virtue of the greater rivet insertion force mayreduce the life of the die, the punch and/or other parts of the rivetingtool.

It has been established that other existing rivets are also notgenerally suitable for joining relatively thick stacks of higherstrength, light metal alloys. The higher strength and low ductility ofsuch material generally means that the rivet experiences higher stressduring the joining operation and this is compounded when a shallow dieis used. Conventional self-piercing rivets are not capable ofwithstanding these high stresses in such a manner that the deformationof the rivet shank remains controllable to ensure the final joint is ofsatisfactory quality. Simply manufacturing the rivet from a higherstrength material does not generally achieve the desired results as thecorresponding reduced ductility can cause the rivet shank to crack as itattempts to deform during insertion. In order to form a suitable jointwith satisfactory strength and corrosion resistance the shank of therivet needs to have sufficient column strength to pierce the top sheetof material without buckling but yet flare outwardly during insertion ina repeatable and predictable manner without tearing or cracking.

One typical approach to strengthening the shank of a self-piercing rivetis to increase its thickness but this increases the tendency of theshank to crack during insertion of the rivet. Another approach is toincrease the depth of rivet material below the head (known as the rivet“web”) thus reducing the length of the unsupported hollow part (thebore) of the shank but this is counter-productive as the volume of sheetmaterial displaced by the rivet is less readily accommodated within thebore leading to the detrimental effects discussed above. The relativelylow ductility of the rivet material only allows for limited deformationand displacement of material before it tends to crack rendering itsusceptible to fatigue. In view of this, self-piercing rivets are notsuccessfully used in thick stack, high strength light alloyapplications. A further approach is to increase the hardness of therivet material but this only increases the tendency of the rivet tofracture as the shank deforms outwardly during the joining process.

It is desirable to produce self-piercing riveted joints in the highstrength materials without having to increase significantly the settingforces required to ensure the rivet is inserted fully. Increase of thesetting forces involves greater energy consumption and thereforeincreased cost and reduces the life of the riveting equipment such as,for example, the punch and the die. Moreover, the commercially availableriveting tools are limited in their setting force capacity. Suchriveting tools are designed to have the capacity to insert manydifferent types of self-piercing rivets into different kinds ofmaterials and are expensive to replace. If such tools were replaced withdesigns that were capable of producing higher forces (whilst providingacceptable performance in other respects) the replacement tools wouldvery likely have to be larger and heavier requiring the use ofreplacement robots with the capacity to handle the increased load. Suchrobots would be more expensive thereby compelling automotivemanufacturers to design more costly production lines that take upgreater space. It is therefore desirable that joints are produced withsetting force magnitudes that are within the capacity of existing rivetsetters (typically around 50 kN).

It is one object of the present invention to obviate or mitigate theaforesaid disadvantages. It is also an object of the present inventionto provide for an improved or alternative method for forming a jointwith a self-piercing rivet.

According to a first aspect of the present invention there is provided amethod for forming a joint in a stack of at least two sheets ofmaterial, at least one of the sheets being a light metal alloy, using aself-piercing rivet comprising the steps of: positioning the materialover a die; providing a substantially cylindrical self-piercing rivetthat is fully hollow so as to define a bore that extends along itsentire length, the rivet having been coated at least along a portion ofits bore by a lubricant; positioning the rivet over the sheet materialat a position opposite the die; using a punch to set the rivet and forceit into the sheet material such that it pierces the upper surfacethereof and such that a shank of the rivet deforms outwardly tointerlock with the material but without penetration to the die side ofthe material; wherein the rivet has a shank with an outside diameter of5.4 mm or less, and the die has a volume that is 60% or less of theeffective solid volume of the rivet.

The invention thus enables the use of lower volume dies with light metalalloys that have relatively low ductility. The use of a lubricantensures that, when such lower volume dies are used the joints can stillbe manufactured using conventional setting force magnitudes even whenthe rivet has a countersunk head. A countersunk head is one that isdesigned to be embedded in the upper sheet of material so that the uppersurface of the rivet head lies substantially flush with the uppersurface of the upper sheet of material in the finished joint. Whensetting such a rivet a significant proportion of the setting force isrequired at the end of the rivet insertion process to ensure the head isembedded in the upper sheet as described.

In one embodiment the outside diameter of the rivet shank may be 5.4 mm,5.3 mm or 5.2 mm. The diameter of the bore may be at least 3.1 mm andmay be 3.2 mm or 3.3 mm. This provides for a rivet with a relativelythin shank wall and therefore a relatively low (but adequate) columnstrength.

In another embodiment the outside diameter of the rivet shank may be3.35 (+/−0.1 mm) and the diameter of the bore may be 2.1 mm (+/−0.1 mm).Such a rivet may be particularly suited for use with a substantiallyflat die.

The sheets may have an ultimate tensile strength in the range 50-600 MPaand preferably in the range 180 MPa-600 MPa. For example they may be ahigh strength aluminium alloy having an ultimate tensile strength in therange 330-600 MPa. Alternatively they may be wrought magnesium alloyhaving an ultimate tensile strength in the range 180-440 MPa. It will beappreciated that the sheets of the joint may be made from differentmaterials, with at least one of the sheets being a light metal alloy oreach sheet in the stack being a light metal alloy but not necessarilythe same as the other sheets in the stack.

The sheets may be produced by conventional rolling, casting or extrusionprocesses combined with hardening or strengthening processes such asheat treatment or age hardening.

The die may have a die cavity with a maximum depth in the range 0.5 mmto 2.0 mm.

The stack of sheet material may have a thickness of at least 6.0 mm, orof at least 3.0 mm, or of at least 1.0 mm.

The rivet may be coated with lubricant along the full length of itsbore. It may also be coated on at least part of the exterior surface ofa shank of the rivet. The lubricant may be of any suitable substancesuch as, for example, a dry film.

The lubricant may comprise a binder resin and one or more suitablelubricant materials such as, for example, graphite, molybdenumdisulphide, PTFE or phosphate.

The lubricant may be applied on an internal surface of the rivet thatdefines the bore and/or may be applied to an external surface of theshank. The lubricant may be applied in any convenient manner. Forexample, it may be inserted into the bore from one end in a droplet orspray form. The amount may be metered so that it is not necessary toadopt a process for removing excess lubricant. In an alternativeexample, the lubricant may be applied by dipping all or part of therivet into a lubricant reservoir. The rivet may be dipped at one endonly. The dipping process may serve to coat one or both of the interiorsurface (which defines the bore) and the external surface of the rivet.

It will be understood that the rivet may be of fastener grade steel suchas, for example, carbon-manganese boron steel confirming to BS EN10263:2001, steel grade 36MnB4. The rivet may have a hardness of 250-600Hv. In a preferred embodiment it has a hardness in the range 280-560 Hv.

The rivet may have a head with a diameter that is larger than that ofthe shank. The head may be designed to finish flush with the top surfaceof the uppermost sheet in the stack of material (e.g. a countersunk headrivet) or it may be designed to stand proud from that surface (e.g. apan-head rivet).

The method may further comprise allowing a slug of the uppermost sheetof material to deform as it reaches the top of the bore, the materialbeing deformed such that it is directed outwardly towards an upperportion of the surface defining the bore. The upper portion of thesurface may exhibit a groove or any other suitable recess or featurewith which the slug engages and thereby inhibits any tendency for theslug to pass out of the top of the rivet. There may be a projection onthe end of the punch for further deforming the slug of material. Thegroove or recess may be annular or partly annular.

According to a second aspect of the present invention there is provideda method for manufacturing a component or product, such as for example acar body panel, including forming a joint in accordance with the methodof forming a joint as described above.

According to a third aspect of the present invention there is provided amethod for forming a joint in a stack of at least two sheets material,at least one of the sheets being a light metal alloy, using aself-piercing rivet comprising the steps of: positioning the materialover a die; providing a substantially cylindrical self-piercing rivetthat is fully hollow so as to define a bore that extends along itsentire length, the rivet having been coated at least along a portion ofits bore by a lubricant; positioning the rivet over the sheet materialat a position opposite the die; using a punch to set the rivet and forceit into the sheet material such that it pierces the upper surfacethereof and such that a shank of the rivet deforms outwardly tointerlock with the material but without penetration to the die side ofthe material; wherein the die has a volume that is 70% or less of theeffective solid volume of the rivet.

The die volume may be 60% or less of the effective solid volume of therivet.

This aspect of the present invention may be combined with any of thefeatures referred to above.

At least one of the sheets may be made from a polymeric or plasticsmaterial.

According to a fourth aspect of the present invention there is provideda method for manufacturing a component or product, such as for example acar body panel, including forming a joint in accordance with the methodof the third aspect of the present invention.

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1 is a sectioned view of a riveted joint using a self-piercingrivet produced according to the method of the present invention;

FIGS. 2a to 2e show sectioned and plan views of prior art joints usingpartially hollow self-piercing rivets, for the purposes of comparisonwith the joint of FIG. 1;

FIGS. 3a and 3b show sectioned views through two riveted joints preparedfor comparative purposes: one using a self-piercing riveting methodaccording to the prior art and the other using a self-piercing rivetmethod in accordance with a second embodiment of the present invention;

FIGS. 4a-4d show plan view and sectioned views for two riveted jointsprepared for comparative purposes: FIGS. 4a and 4b showing aself-piercing rivet method according to the prior art and the FIGS. 4cand 4d using a self-piercing rivet method in accordance with a thirdembodiment of the present invention; and

FIG. 5a is a sectioned view of a joint in accordance with the presentinvention showing the upper portion of a rivet designed to prevent orinhibit a slug of material inside the rivet bore from become dislodged;

FIG. 5b is a magnified view of an upper portion of the joint of FIG. 5a; and

FIG. 5c is a schematic representation of an embodiment of a method forproducing the joint of FIGS. 5a and 5 b.

Referring now to FIG. 1 of the drawings, there is shown a joint formedin a stack of three sheets of high strength aluminium alloy (6111-T4) bya self-piercing rivet.

In this particular example, the rivet has been inserted using anelectric rivet setter of the kind described in U.S. Pat. No. 6,951,052and using a clamping regime described in U.S. Pat. No. 6,742,235. Thejoint has been formed using a die with a maximum depth of 1.8 mm andhaving a diameter of 10 mm. The setting velocity was 350 mm/sec.

The rivet is substantially cylindrical and fully hollow, that is, it hasa central bore that it open at both ends, an outside diameter of 5.3 mm(+/−0.1 mm) and an inside diameter of 3.2 mm (+/−0.1 mm). At its upperend the rivet has countersunk head defined by an annular projection atits outer periphery to provide an increased diameter of 7.75 mm thusproviding sufficient area for contact by a driving punch of the rivetsetter. The inside of the bore is mechanically plated at the head end toa short distance of approximately 0.5 to 1 mm. The bore is also coated,along its full length, with a dry film lubricant by dipping the rivet inthe lubricant in liquid form, spinning the rivet in a centrifuge or thelike to remove excess liquid and allowing the lubricant to dry as afilm. One example of a commercially available film lubricant is Gleitmo625 available from Fuchs Lubritech GmbH of Kaiserslauten, Germany.

From a simple visual inspection of the image of FIG. 1 it can be seenthat a satisfactory joint has been produced. In particular, the rivetdemonstrates sufficient column strength and exhibits no asymmetry orpartial collapse. The rivet has pierced through the upper andintermediate sheets but not the lowermost sheet with which it hasinterlocked sufficiently so as to produce a robust joint. The degree ofinterlock is represented by the measurement of 0.44 mm shown in FIG. 1.There is no evidence of the second sheet pushing through the lowermostsheet. The thickness of the lowermost sheet in the joint is shown as0.71 mm, which is considered a satisfactory depth. It can be seen thatthe aluminium has flowed to the top of the bore. The lubricant hasserved to reduce the stresses encountered by the rivet in the settingoperation induced by friction thereby allowing the sheet material toflow to the top of the bore. The “button” of material that is formedinto the die has not torn or cracked during the rivet insertion process.The result is not only a joint that is aesthetically acceptable but alsoensures that the exposed part of the bore at the head end is restrictedto a depth where the plating provides corrosion protection. In theabsence of a lubricant it was found that the same rivet would experiencegreater stress and would tend to become compressed resulting in anasymmetric form in the final joint. In addition, the sheet materialwould only flow part way up the rivet bore thereby rendering the jointsusceptible to corrosion.

The use of a fully hollow rivet of this kind affords a relatively largebore volume that can accommodate a larger percentage of displaced sheetmaterial. This allows the use of a relatively shallow die that reducesthe tendency for the intermediate sheets to be dragged down and throughthe lowermost sheet. Moreover, the tendency of such sheets to wraparound the piercing end of the rivet is reduced. In the particularexample shown in FIG. 1, the fully hollow rivet is capable ofaccommodating 81% more displaced sheet material than a conventionalself-piercing rivet.

With an outer diameter of 5.3 mm and an inside diameter of 3.2 mm, thetotal shank thickness across the diameter of the rivet of FIG. 1 is 2.1mm, i.e. the radial thickness of the shank wall is 1.05 mm. Such arivet, with no web of material closing the bore, has a significantlyreduced column strength, which had been considered insufficient for usewith thick stack high strength aluminium or other high strength lightmetal alloys in conjunction with low volume dies. However it has beenestablished that with the combination of a rivet of such geometry, a lowvolume die and lubrication, a joint of a satisfactory quality can beproduced with surprisingly low setting forces. The lower forces alsomean that the possibility of the material of the lower sheet becomingembedded and retained in the die is reduced.

It will be appreciated that the depth and diameter of the die may bevaried depending on the rivet size. In particular the die may have a diecavity with a maximum depth in the range 0.5 mm to 2.0 mm. It has beenestablished that the die may have a volume this is 60% or even 70% orless of the effective solid volume of the rivet.

For the purposes of comparison with the joint of FIG. 1, FIGS. 2a to 2fshow joints that have been produced in the same type and thickness ofsheet material with a conventional, partly hollow, rivet in which thebore extends only part-way long the rivet shank so as to leave a web ofmaterial under the rivet head.

In FIG. 2a the rivet is of the same length and hardness as that ofFIG. 1. The joint has been produced in the same manner as that of FIG.1, with the same die and setting velocity. It can be seen that the rivetdoes not have sufficient column strength as it has flared asymmetricallyin the joint. It has been established that this is as a result of thebore becoming prematurely full of the sheet material during the rivetinsertion process such that the rivet flares and compresses. Thisresults in a significantly reduced interlock, particularly on the lefthand side in this example. The intermediate sheet has pushed through thelowermost sheet as indicated by arrow A thereby providing a path formoisture ingress and a potential for rivet corrosion. The head is notcompletely flush with the top surface of the upper sheet, whichindicates that, unlike the joint shown in FIG. 1, a greater settingforce is required. However, an increase in the setting force would notcure the problem of insufficient column strength. It will be appreciatedby the skilled person that increasing the setting force is generallyundesirable or impractical. In particular, such an increase involvesgreater energy consumption, the use of potentially larger force capacityriveting tools requiring large robots and greater expense as describedin the introductory part of this specification. Moreover, simplyincreasing the setting force would only serve to increase the tendencyfor the rivet to collapse or flare asymmetrically.

In FIG. 2b the only parameter that has changed compared to the joint ofFIG. 2a is that the die depth has increased to 2.5 mm in an effort toaccommodate more of the displaced sheet material. It can be seen thatthe rivet again has insufficient column strength to withstand the volumeof material pushed into the bore, as it has flared asymmetrically. Theincrease in die depth has resulted in facture of the button (see arrowB) and has not reduced the tendency of the intermediate sheet to pushthrough the lower sheet (see arrow C). Both these defects provide apotential source of corrosion. It has been realised by the inventor thatincreasing the depth of the die does not improve the joint when usingsheet material that has lower ductility such as higher strengthaluminium alloy.

In FIG. 2c the only parameter that has changed compared to the joint ofFIG. 2a is that the hardness level of the rivet has increased. It can beseen that the rivet exhibits increased column strength but someasymmetry is still apparent. The intermediate sheet has once againpushed through the lowermost sheet. The rivet provides minimal interlockin the lowermost sheet and the head stands proud of the upper surface ofthe uppermost sheet, indicating that a greater setting force isnecessary (the undesirability of this will be apparent to the skilledperson as discussed above). This joint is still not consideredsatisfactory for many applications.

Referring now to FIGS. 2d and 2e , the only parameter that has changedcompared to the joint of FIG. 2a is that the rivet exhibits increasedcolumn strength by virtue of its thicker legs. FIG. 2d is a sectionedview as before but FIG. 2e is a plan view of the lowermost sheet (afterthe joint has been cut through to produce the sectioned view of FIG. 2d). The intermediate sheet has been dragged down such that the sheetwraps around the piercing end and is not penetrated (see arrow D). Thisresults in insufficient interlock. The rivet head also remains proud ofthe upper sheet again indicating that a greater setting force isrequired to set the rivet flush. The reduced volume of the bore alsoresults in the intermediate sheet pushing through the lowermost sheet,as can be seen from arrow E in FIG. 2 e.

The present invention also has application to thin stacks of aluminiumalloys. FIG. 3a shows a joint produced in two sheets of aluminium alloyNG5754, each having a thickness of 1.5 mm. In such stacks the vastmajority of the displaced sheet material can be accommodated in the boreof the rivet and so the joint has been produced using a substantiallyflat die, resulting in the absence of a button on the die side of thematerial. The rivet is fully hollow and has an outside shank diameter of3.35 mm (+/−0.1 mm) and a bore diameter of 2.1 mm (+/−0.1 mm). It willbe seen that the joint is virtually flush on both the upper and lowersurfaces, which is particularly desirable for certain joints such as,for example, those that are particularly prominent in the finishedarticle or where out-of-plane deformation of the lowermost sheet and/orthe complete joint cannot be accommodated or would not be visuallyacceptable. The minimum thickness of the lowermost sheet is 0.37 mm,which is considered within the bounds of a satisfactory joint in mostapplications.

For comparison FIG. 3b illustrates a standard joint used in theautomotive industry for such material and that has been produced using acomparable conventional partly hollow rivet. It will be seen that thedie side of the joint has a significant button deformation.

FIGS. 4a-4d show joints that have been produced in wrought magnesiumalloy AZ31 B-0. Such material has a particularly low ductility and istherefore difficult to join using self-piercing riveting methods. Afirst joint, shown in FIGS. 4a (plan view) and 4 b (sectioned view) hasbeen produced using a partly hollow rivet and a die having a depth of1.6 mm. The internal bore of the rivet has been lubricated in the samemanner as the rivet used in the joint of FIG. 1. It can be seen fromarrows F and G that the button exhibits cracking and tearing renderingthe joint prone to corrosion. Moreover, there is a lack of interlock inthe joint rendering it relatively weak. Simply reducing the die volumein an attempt to reduce the size of the button has been found to causerivet collapse or a joint in which the rivet is not flush with the uppersurface of the upper sheet. In contrast a successful joint has beenproduced using a fully hollow rivet with a low volume die of maximumdepth 0.75 mm, as shown in FIGS. 4c and 4d . No cracks or tears in thebutton are evident and the piercing end of the rivet has penetrated theupper sheet with sufficient flaring so as to provide good interlock inthe lower sheet.

The use of a fully hollow rivet in the method of the present inventionpermits the use of a die of significantly lower volume than compared tothat used in conventional self-piercing riveting methods, without fearof the rivet collapsing or flaring in an asymmetric manner. Such lowvolume dies would be considered too small for use with conventionalrivets for joining the same sheet materials. The volume of the die usedin the method of the present invention may have a volume anywherebetween 30% and 100% lower than that used in a conventionalself-piercing riveting process.

The die volume may be anywhere between 0% and 60% or even 70% of anequivalent solid rivet volume (i.e. a rivet having the same dimensionsas the fully hollow rivet used to make the joint of the presentinvention, but which was completely solid). This compares to dies usedin conventional self-piercing riveting processes (with partly hollowrivets) that are typically above 60% of the equivalent solid volumerivet.

In one embodiment, for example, the die may have a maximum depth of 2 mmand a diameter of 10 mm.

Any suitable lubricant may be used such as, for example, a dry (solid)film lubricant. The lubricant may be applied along the full length ofthe rivet bore or just part thereof. It may also be applied, in someapplications, to the exterior surface of the rivet shank.

An advantage of using a fully hollow rivet having a shank with anoutside diameter of 5.3 mm (+/−0.1 mm) is that it may be used withstandard rivet setters, dies, rivet feeders and other tooling used tomake self-piercing rivet joints. This means the same equipment may beused to insert both conventional partly hollow and fully hollow rivets.

Joints in accordance with the present invention may be produced usingsignificantly lower setting forces than those for conventional rivets.Moreover, the rivets provide for a reduced weight in the final assembledproduct and through imparting reduced distortion allow joints to beproduced in restricted areas such as, for example, narrow flanges.

The rivet used in the method of the present invention may have anoutside shank diameter of less than 5.3 mm (+/−0.1 mm). Furthermore itmay be used to join stacks that have a thickness of 3 mm or less,including a stack having a thickness of 1 mm.

The use of a lubricant on the rivet means that when lower volume diesare used the joints can be made at standard or conventional settingforce of less than 50 kN, even when the rivet has a countersunk head.When setting such a rivet a large proportion of the setting force isrequired to embed the underside of the head into the upper sheet therebyensuring that the head is substantially flush with the upper surface ofthe upper sheet (as seen in FIG. 1).

The term “light metal alloys” is recognised in the industry and is usedherein to mean magnesium, titanium, beryllium or high strength aluminiumalloys, which all have a low density and high strength to weight ratios.

The ultimate tensile strength (UTS) of wrought magnesium alloys istypically in the range of 180-440 MPa, whereas the UTS of high strengthaluminium alloys is typically in the range 330-600 MPa.

FIGS. 5a-5c illustrate how a slug 11 of the upper sheet material isretained in the bore of the rivet. FIG. 5b is a magnified view of theupper part of FIG. 5a and shows that the internal surface of the rivethas an annular groove 12 defined at its upper end. The slug has deformedoutwardly to occupy the groove thereby preventing it from becomingdislodged from the upper end of the bore. In the embodiment shown thegroove is annular but will be appreciated that the groove may be partlyannular or may take any other suitable form.

In this exemplary embodiment the joint has been produced using a stackof four sheets of high strength aluminium alloy AC600-T4 material, theupper sheet being relatively thin (0.9 mm). The next sheet down is 2 mmthick and the other two sheets are 3 mm thick. As before, the rivet hasbeen inserted using an electric rivet setter of the kind described inU.S. Pat. No. 6,951,052 and using a clamping regime described in U.S.Pat. No. 6,742,235. The joint has been formed using a die with a maximumdepth of 1.8 mm and having a diameter of 10 mm. The setting velocity was350 mm/sec.

FIG. 5c shows a joint produced with the same rivet with a punch 13 (thatinserts the rivet) that has a projection 14 to encourage deformation ofthe slug material into the groove 12.

In some applications the die may have a volume that is 60-70% of theeffective solid volume of the rivet.

The invention claimed is:
 1. A method for forming a joint in a stack ofat least two sheets of material, at least one of the sheets being alight metal alloy, using a self-piercing rivet comprising the steps of:positioning the material over a die; providing a substantiallycylindrical self-piercing rivet that is fully hollow so as to define abore that extends along its entire length, the rivet having been coatedat least along a portion of its bore by a lubricant; positioning therivet over the sheet material at a position opposite the die; using apunch to set the rivet and force it into the sheet material such that itpierces an upper surface thereof and such that a shank of the rivetdeforms outwardly to interlock with the material but without penetrationto the die side of the material; wherein the rivet has a shank with anoutside diameter of 5.4 mm or less, and the die has a volume that is 60%or less of an effective solid volume of the rivet.
 2. A method accordingto claim 1, where the sheets have an ultimate tensile strength in therange of 50-600 MPa.
 3. A method according to claim 1, wherein thesheets have an ultimate tensile strength in the range of 180 MPa-600MPa.
 4. A method according to claim 1, wherein the die has a die cavityhaving a maximum depth in the range of 0.5 mm to 2.0 mm.
 5. A methodaccording to claim 1, in which the stack of sheet material has athickness of at least 6.0 mm.
 6. A method according to claim 1, in whichthe stack of sheet material has a thickness of at least 1.0 mm.
 7. Amethod according to claim 1, wherein the rivet is coated with lubricantalong a full length of its bore.
 8. A method according to claim 7,wherein the rivet is further coated with lubricant on at least part ofan exterior surface of a shank of the rivet.
 9. A method according toclaim 1, wherein the lubricant is a dry film.
 10. A method according toclaim 1, wherein the sheet material is high strength aluminum alloyhaving an ultimate tensile strength in the range of 330-600 MPa.
 11. Amethod according to claim 1, wherein the sheet material is wroughtmagnesium alloy having an ultimate tensile strength in the range of180-440 MPa.
 12. A method according to claim 1, wherein a diameter ofthe bore of the rivet is at least 3.1 mm.
 13. A method according toclaim 1, wherein an outside diameter of the shank is 3.36 mm or less.14. A method according to claim 13, wherein an inside diameter of thebore of the rivet is at least 2.0 mm.
 15. A method according to claim 1,wherein the rivet has a countersunk head.
 16. A method according toclaim 1, wherein the rivet has at least one formation at an upperportion of the bore, the method including allowing the sheet material todeform such that a slug of material from the upper sheet and inside thebore engages with said at least one formation.
 17. A method according toclaim 1, wherein the at least one formation comprises a groove orrecess.
 18. A method according to claim 17, further comprising using apunch to insert the rivet into the sheet material, the punch having aprojection that contacts the slug of material so as to deform itoutwardly.
 19. A method for manufacturing a component or productincluding forming a joint in accordance with the method of claim
 1. 20.A method for forming a joint in a stack of at least two sheets material,at least one of the sheets being a light metal alloy, using aself-piercing rivet comprising the steps of: positioning the materialover a die; providing a substantially cylindrical self-piercing rivetthat is fully hollow so as to define a bore that extends along itsentire length, the rivet having been coated at least along a portion ofits bore by a lubricant; positioning the rivet over the sheet materialat a position opposite the die; using a punch to set the rivet and forceit into the sheet material such that it pierces an upper surface thereofand such that a shank of the rivet deforms outwardly to interlock withthe material but without penetration to the die side of the material;wherein the die has a volume that is 70% or less of an effective solidvolume of the rivet.
 21. A method according to claim 20, wherein the dievolume is 60% or less of the effective solid volume of the rivet.
 22. Amethod for manufacturing a component or product including forming ajoint in accordance with the method of claim 20.